Problem: If $\angle LON$ is a right angle and $m \angle LOM = 78^\circ$, what is $m \angle MON$, in degrees? $78^\circ$ $O$ $L$ $N$ $M$
Answer: From the diagram, we see that $\angle LOM$ and $\angle MON$ are complementary angles. Therefore, $m \angle LOM + m \angle MON = 90^\circ$ Thus, $m \angle MON = 90^\circ - m \angle LOM = 90^\circ - 78^\circ = 12^\circ$.